The comments of Gutmann et al are greatly appreciated. They provide an opportunity to explain in greater detail the calculations that led us to conclude that children with neurofibromatosis type 1 (NF1) and juvenile xanthogranuloma (JXG) have a greater risk for juvenile chronic myelogenous leukemia (JCML) than children with NF1 who do not have JXG.1

Several statements by Gutmann and colleagues deserve clarification. First, regarding the frequency of NF1, these authors choose to cite 1 study,2 while other population-based studies3-5 have shown a wider range of frequency of the disease (1 per 1000 to 1 per 4600). Second, Gutmann and colleagues question the frequency of JCML used in our study by citing the lower annual incidence rate of the disease. The frequency of a disease reflects the number of affected persons in a particular population over a lifetime. Thus, it cannot be used interchangeably with the annual